Cycles - Adding an object with emission shader makes part of my scene darker?
What's happening here, is you have your light inside. Cycles is throwing a lot of samples there because there aren't any other lights. Then, you added your other light and it had multiple importance enabled.

Cycles will of coarse have to throw some samples at that light, so that in it's self will make it noisier, but in addition to that, having multiple importance on, tells Cycles that that light is more important than the other light, so it throws even more samples at it.

So basically what you're seeing, is your render getting noisier, and therefore appearing darker, because of the extra lights.